Method of manufacturing circuit board

ABSTRACT

A pre-preg sheet includes a substrate and a resin impregnated in the substrate. A first metal foil is placed on the pre-preg sheet to provide a laminated body. The laminated body is put in a heating device having a temperature maintained at a temperature close to a softening temperature of the resin. The laminated body is compressed at the temperature at a predetermined pressure. The first metal foil is bonded to the pre-preg sheet of the laminated body and hardening the resin, thus providing a circuit board. This method provides a stable resistance of a conductive paste filing a through-hole in the pre-preg sheet to be compressed at a small rate.

DESCRIPTION Technical Field

The present invention relates to a method of manufacturing a circuitboard having circuit patterns on both surfaces.

BACKGROUND OF THE INVENTION

As electronic instruments have recently been small and had highdensities, multi-layer circuit boards have been required not only inindustrial instruments but also in consumer instruments.

Such circuit boards require a method of interconnecting circuit patternson plural layers through an inner via-hole and a highly-reliablestructure. Japanese Patent Laid-Open Publication No.6-268345 discloses aconventional method of manufacturing a multi-layer circuit board havingan inner via-hole made of conductive paste. The conventional method ofmanufacturing a circuit board having four layers will be described.

First, a method of manufacturing a double-sided circuit board used as asubstrate of the multi-layer circuit board will be described. FIG. 6A toFIG. 6G are sectional views showing processes of the conventional methodof manufacturing the double-sided circuit board.

Pre-preg sheet 101, a substrate, is made of a composite materialincluding a core and thermosetting epoxy resin impregnated in the core.The core is made of non-woven fabric, such as aromatic polyamide fiber,and has a thickness t101 of 150 μm compressed at a compression rate ofabout 35%. Pre-preg sheet 101 employs a porous material having vacancyfor obtaining a compressive property.

FIG. 6A shows pre-preg sheet 101 having both surfaces onto whichreleasing films 102 a and 102 b are bonded, respectively. Respectivesurfaces of releasing films 102 a and 102 b are coated with Si-basedreleasing agent. The films are made of film, such as polyethyleneterephthalate film. Through-hole 103 is formed in predeterminedpositions of pre-preg sheet 101 by a laser machining method, as shown inFIG. 6B. Through-hole 103 are filled with conductive paste 104 by aprinting method, as shown in FIG. 6C.

Then, releasing films 102 a and 102 b are peeled off from the surfacesof pre-preg sheet 101, as shown in FIG. 6D. Metal foils 105 a and 105 bare placed on the surfaces of pre-preg sheet 101, and are heated andpressurized by hot press, as shown in FIG. 6E. Thus, thickness t102 ofpre-preg sheet 101 is reduced to about 100 μm, and pre-preg sheet 101 isbonded to metal foils 105 a and 105 b, as shown in FIG. 6F. Metal foils105 a and 105 b are electrically connected via conductive paste 104filling through-hole 103.

Metal foils 105 a and 105 b is selectively etched to form circuitpatterns 106 a and 106 b, thus providing a double-sided circuit boardshown in FIG. 6G.

FIG. 7A to FIG. 7D are sectional views showing the conventional methodof manufacturing a multi-layer circuit board having four layers as willbe described below.

First, as shown in FIG. 7A, double-sided circuit board 110 havingcircuit patterns 106 a and 106 b is prepared, and pre-preg sheets 101 aand 101 b having through-holes 103 filled with conductive paste 104 areprepared. Double-sided circuit board 110 is manufactured by theprocesses shown in FIG. 6A to FIG. 6G, and pre-preg sheets 101 a and 101b are manufactured by the processes shown in FIG. 6A to FIG. 6D.

Then, metal foil 105 b, pre-preg sheet 101 b, double-sided circuit board110, pre-preg sheet 101 a, and metal foil 105 a are positioned andstacked in this order on a laminated plate (not shown), as shown in FIG.7B.

Then, metal foil 105 b, pre-preg sheet 101 b, double-sided circuit board110, pre-preg sheet 101 a, and metal foil 105 a are heated andpressurized by hot press. Thus, as shown in FIG. 7C, pre-preg sheets 101a and 101 b are compressed to have thickness t102, and are bonded todouble-sided circuit board 110 and metal foils 105 a and 105 b. Circuitpatterns 106 a and 106 b are electrically connected to metal foils 105 aand 105 b via conductive paste 104, respectively.

As shown in FIG. 7D, metal foils 105 a and 105 b are selectively etchedto form circuit patterns 106 a and 106 b, thus providing four-layercircuit board 120.

A multi-layer circuit board having more than four layers, such as asix-layer circuit board, is obtained by repeating the processes shown inFIG. 7A to FIG. 7D using four-layer circuit board 120 obtained by theprocesses of FIG. 7A to FIG. 7D instead of double-sided circuit board110.

In the case that the through-hole has a small diameter and is arrangedadjacent to another through-hole by a small pitch for providing a finecircuit board, the conventional method of manufacturing the circuitboard has the following problem.

The pre-preg sheet made of porous material has a vacancy to becompressed. When the volume ratio of the vacancy to the pre-preg sheetis large, a portion of the conductive paste intends to be put into thevacancy. The resistance of the conductive paste in the hole accordinglyincreases, and electrical insulation between the conductive pastes inthe adjacent through-hole may be hardly obtained. Therefore, a materialhaving small porosity may preferably used, but the material having thesmall porosity cannot be compressed at a high compressed rate.

FIG. 8A and FIG. 8B are sectional views of the circuit board formed bythe conventional method.

FIG. 8A shows pre-preg sheet 101 having a compression rate of 35%. InFIG. 8A, pre-preg sheet 101 is sufficiently compressed before resinimpregnated into pre-preg sheet 101 flows in surface direction D101, sothat conductive paste 104 does not flow out of through-hole 103 and hasa stable resistance.

In FIG. 8B, pre-preg sheet 101 has a high porosity and a smallcompressed rate, e.g. smaller than 10%. A compression rate of conductivepaste 104 decreases during heating and pressurizing, and conductivepaste 104 may flow as denoted by flow 115. A contacting force betweenconductive particles in conductive paste 104 accordingly decreases.

When the resin in pre-preg sheet 101 melts due to the heating andpressurizing and flows in surface direction D101, conductive paste 104flows out of through-hole 103. The contacting force between theconductive particles in conductive paste 104 accordingly decreases thusincreasing the resistance of a portion of conductive paste 104 inthrough-hole 103. Then, the connection resistance between metal foils105 a and 105 b accordingly increases, thus causing a quality of thecircuit board to decline.

In order to solve this problem, the metal foil and a pre-preg sheethaving a core and a resin impregnated into the core are stacked, andthey are then heated for a predetermined time at a first temperatureclose to a softening temperature of the resin while being pressurized bya predetermined pressure, and then heated for a predetermined time at asecond temperature higher than the first temperature and pressurized.

In this method, the processes at the first temperature to the secondtemperature are executed continuously, so that the rate of a temperaturerise during the processes is restricted. Specifically, when the firsttemperature varies to the second temperature, the rate of thetemperature rise at a temperature at which the resin in the pre-pregsheet melts and flows may be reduced due to a delay of heat conductionthrough an intermediate material, such as a cushioning material or a SUSplate. The rate of the temperature rise may not reach a predeterminedrate. In other words, fluidiity of the resin during molding is notsufficiently secured, so that the pre-preg sheet can hardly be moldedespecially when the viscosity of the melting resin is high.

SUMMARY OF THE INVENTION

A pre-preg sheet including a substrate and a resin impregnated in thesubstrate is provided. A first metal foil is placed on the pre-pregsheet to provide a laminated body. The laminated body is put in aheating device having a temperature maintained at a temperature close toa softening temperature of the resin. The laminated body is compressedat the temperature at a predetermined pressure. The first metal foil isbonded to the pre-preg sheet of the laminated body and hardening theresin, thus providing a circuit board.

This method provides a stable resistance of a conductive paste filling athrough-hole in the pre-preg sheet to be compressed at a small rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a sectional view of a double-sided circuit board for showinga method of manufacturing the board in accordance with an exemplaryembodiment of the present invention.

FIG. 1B is a sectional view of the double-sided circuit board forshowing the method of manufacturing the board in accordance with theembodiment.

FIG. 1C is a sectional view of the double-sided circuit board forshowing the method of manufacturing the board in accordance with theembodiment.

FIG. 1D is a sectional view of the double-sided circuit board forshowing the method of manufacturing the board in accordance with theembodiment.

FIG. 1E is a sectional view of the double-sided circuit board forshowing the method of manufacturing the board in accordance with theembodiment.

FIG. 1F is a sectional view of the double-sided circuit board forshowing the method of manufacturing the board in accordance with theembodiment.

FIG. 1G is a sectional view of the double-sided circuit board forshowing the method of manufacturing the board in accordance with theembodiment.

FIG. 1H is a sectional view of the double-sided circuit board forshowing the method of manufacturing the board in accordance with theembodiment.

FIG. 2A shows a method of measuring a resin state of a pre-preg sheet inaccordance with the exemplary embodiment.

FIG. 2B shows the state of a resin in a pre-preg sheet in accordancewith the embodiment.

FIG. 3 shows a profile of the pre-preg sheet upon being compressed inaccordance with the embodiment.

FIG. 4 shows another profile of the pre-preg sheet upon being compressedin accordance with the embodiment.

FIG. 5 shows a further profile of the pre-preg sheet upon beingcompressed in accordance with the embodiment.

FIG. 6A is a sectional view of a double-sided circuit board for showinga conventional method of manufacturing the board.

FIG. 6B is a sectional view of the double-sided circuit board forshowing the conventional method of manufacturing the board.

FIG. 6C is a sectional view of the double-sided circuit board forshowing the conventional method of manufacturing the board.

FIG. 6D is a sectional view of the double-sided circuit board forshowing the conventional method of manufacturing the board.

FIG. 6E is a sectional view of the double-sided circuit board forshowing the conventional method of manufacturing the board.

FIG. 6F is a sectional view of the double-sided circuit board forshowing the conventional method of manufacturing the board.

FIG. 6G is a sectional view of the double-sided circuit board forshowing the conventional method of manufacturing the board.

FIG. 7A is a sectional view of a four-layer circuit board for showingthe conventional method of manufacturing the board.

FIG. 7B is a sectional view of the four-layer circuit board for showingthe conventional method of manufacturing the board.

FIG. 7C is a sectional view of the four-layer circuit board for showingthe conventional method of manufacturing the board.

FIG. 7D is a sectional view of the four-layer circuit board for showingthe conventional method of manufacturing the board.

FIG. 8A is a sectional view of the circuit board manufactured by theconventional method.

FIG. 8B is a sectional view of the circuit board manufactured by theconventional method.

REFERENCE NUMERALS 1  Pre-preg Sheet 1a Pre-preg Sheet 1b Pre-preg Sheet2a Releasing Film 2b Releasing Film 3  Through-Hole 4  Conductive Paste5a Metal Foil 5b Metal Foil 6a Circuit Pattern 6b Circuit Pattern 6cCircuit Pattern 6d Circuit Pattern

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1A to FIG. 1G are sectional views of a double-sided circuit boardfor showing a method of manufacturing the board in accordance with anexemplary embodiment of the present invention.

Pre-preg sheet 1 is made of a composite material substrate 1 a of afiber sheet, such as non-woven fabric of aromatic polyamide fiber andthermosetting epoxy resin 1 b impregnated into the non-woven fabric. Thenon-woven fabric has a size of 250 mm by 250 mm and has a thickness ofabout 110 μm. The epoxy resin has a softening temperature of 70° C. anda minimum melting viscosity of 1000 Pa·s. The composite material isheated and pressurized by hot press to have a thickness of about 100 μm,and the resin is at a B stage (semi-hard state).

Conductive paste 4 filling through-hole 8 is produced by sufficientlykneading three rolls of 85 wt. % of conductive filler, 12.5 wt. % ofthermosetting epoxy resin having no solvent, and 2.5 wt. % of anhydridehardening agent.

The conductive filler is Cu powder having an average grain size of 2 μm,and may be powder of Au, Ag, or an alloy of them.

The viscosity of the thermosetting epoxy resin at softening and meltingis the smallest at temperatures lower than 70° C.

The softening temperature of the thermosetting epoxy resin contained inconductive paste 4 is preferably lower than that of the thermosettingepoxy resin impregnated into pre-preg sheet 1.

As shown in FIG. 2A, resin 52 was put into cell 51. Resin 52 was heatedwhile a pressure of 5 MPa was applied to resin 52 with piston 53.Displacement 54 of piston 53 provided when resin 52 was softened andflows due to the heating was measured.

FIG. 2B shows displacement 54 of piston 53. Temperature range R1 fromsoftening temperature T1 (65° C.) to melting temperature T2 (85° C.) ofresin 52 is a softening range of resin 52, and the resin is soft buthardly flows within this range. In this softening range, pre-preg sheet1 is easily compressed, so that temperature range R1 is adequate for acompressing process. The temperature range from melting temperature T2(85° C.) to 140° C. is flowing/hardening range R2 when resin 52 flows toproceed a hardening reaction. Since the resin flows, this temperaturerange is adequate for a molding process.

The method of manufacturing the circuit board in accordance with theembodiment will be described below. Processes shown in FIG. 1A to FIG.1E are the same as conventional processes shown in FIG. 6A to FIG. 6E,respectively, and the descriptions of FIG. 1A to FIG. 1E are omitted.

COMPARATIVE EXAMPLE 1

As shown in FIG. 6E, copper foils 105 a and 105 b having thicknesses of18 μm were placed on pre-preg sheet 101 at a room temperature to providelaminated body 181. Ten sets of laminated bodies 181 were installed on astainless mirror plate having a thickness of about 1 mm over a settingplate.

As shown in FIG. 6F, laminated body 181 was then mounted on a vacuum hotpressing machine, heated, and pressurized. FIG. 3 shows temperatureprofile Tp1 and temperature Tb1 of the laminated body at this moment. Atemperature-rising rate of the resin of laminated body 181 withinflowing/hardening range R2 was determined to be 5° C./min., andlaminated body 181 was pressurized at a pressure of 5 MPa in range R1 tobe molded.

COMPARATIVE EXAMPLE 2

As shown in FIG. 6E, copper foils 105 a and 105 b having thicknesses of18 μm were placed on pre-preg sheet 101 at a room temperature to providelaminated body 181. Ten sets of laminated bodies 181 were installed on astainless mirror plate having a thickness of about 1 mm over a plate.

As shown in FIG. 6F, laminated body 181 was then mounted on a hotpressing machine, heated, and pressurized. FIG. 4 shows temperatureprofile Tp2 and temperature Tb2 of the laminated body at this moment.Laminated body 181 was pressurized at a pressure of 5 MPa in resinsoftening range R1 to be molded.

A temperature-rising rate of temperature profile Tp2 inflowing/hardening range R2 was 5° C./min., namely, the same as that oftemperature profile Tp1 shown in FIG. 3, but the temperature-rising rateof temperature Tb2 of laminated body 181 was 3° C./min.

EXAMPLE 1

As shown in FIG. 1E, copper foils 5 a and 5 b having thicknesses of 18μm were placed on pre-preg sheet 1 at a room temperature to providelaminated body 81. Ten sets of laminated bodies 81 were placed on astainless mirror plate having a thickness of about 1 mm over a plate.

Laminated bodies 81 over the plate were put into a hot pressing machine,pressurized at a pressure of 5 MPa for 10 minutes, and then taken out.Here, the hot pressing machine was a heating/pressurizing device havinga temperature maintained at 70° C.

Next, as shown in FIG. 1F, laminated body 81 was heated according totemperature profile Tp1 shown in FIG. 3 by a vacuum hot pressingmachine, pressurized at a pressure of 5 MPa within resinflowing/hardening range R2 to be molded.

Laminated body 81 may be heated at 80° C. by a dryer as the heatingdevice and put into the vacuum hot pressing machine.

By this method, a predetermined temperature-rising rate of laminatedbody 81 was obtained by even if temperature rising delayed due to anintermediate material, such as the stainless plate, and the hot pressingmachine as a heating/pressurizing device.

EXAMPLE 2

As shown in FIG. 1E, copper foils 105 a and 105 b having thicknesses of18 μm were placed on pre-preg sheet 1 to provide laminated body 81.Then, as shown in FIG. 1H, each of 10 sets of laminated bodies 81 wassandwiched between stainless mirror plates 83 having thicknesses ofabout 1 mm, and was installed on a plate. Stainless mirror plates 83, alaminating device, were previously heated at 80° C. by a dryer.

Then, laminated body 81 was put into a vacuum hot pressing machine andheated according to temperature profile Tp1 shown in FIG. 3, and waspressurized at a pressure of 5 MPa within resin flowing/hardening rangeR2 to be molded.

By this method, a predetermined temperature-rising rate of in laminatedbodies 81 was obtained even if the temperature rising was delayed due tothe stainless plate as the heating device or an intermediate material.

EXAMPLE 3

As shown in FIG. 1E, copper foils 5 a and 5 b having thicknesses of 18μm were placed on pre-preg sheet 1 at a room temperature to provide 10sets of laminated bodies 81. Laminated body 81 was placed on a stainlessmirror plate having a thickness of about 1 mm.

Laminated body 81 was then put into a vacuum hot pressing machine,heated, and pressurized. FIG. 5 shows temperature profile Tp3 andtemperature Tb3 of laminated body 81 at this moment. A temperature oflaminated body 81 was maintained at 70° C. within resin softening rangeR1 for 10 minutes, compressed at 5 MPa, and then cooled to a temperaturenot higher than 50° C. The temperature-rising rate of temperatureprofile Tp3 in resin flowing/hardening range R2 was determined to be 5°C./min.

FIG. 3 to FIG. 5 show only temperature profiles Tp1, Tp2, and Tp3 andtemperatures Tb1, Tb2 and Tb3 of laminated bodies 81 and 181, andexclude temperature profiles, pressure profiles, and vacuum pressures inthe resin hardening range and during cooling. Within resin hardeningrange R2, the laminated bodies of all of comparative examples 1 and 2and examples 1 to 3 were maintained at 200° C. for about 60 minutes toharden the resins, and then cooled.

During a process of compressing laminated bodies 81 and 181 of examples1 to 3 and comparative example 2, the thermosetting epoxy resin in theconductive paste is softened, and has a viscosity reaching the smallestvalue. Thus, conductive paste 4 can deform by any pressure and becompressed slowly.

A contacting force between Cu particles in the conductive paste canincrease when the thermosetting epoxy resin (including no solvent)diffuses from through-hole 3 to the copper foils.

After laminated bodies 81 and 181 of examples 1 to 3 and comparativeexample 2 were compressed, copper foils 5 a, 5 b, 105 a and 105 b werepeeled off for observation of pre-preg sheets 1 and 101. It wasconfirmed that resins in conductive pastes 4 and 104 diffuse in copperfoils 5 a, 5 b, 105 a, and 105 b, and that pre-preg sheets 1 and 101were slightly molded to have thicknesses reduced.

The resistances of conductive pastes 4 and 104 in through-holes 3 and103 in the double-sided circuit boards (FIG. 1G) and the four-layercircuit boards manufactured by the methods of examples 1 to 3 andcomparative example 2 are about 20% smaller than that of the circuitboard of comparative example 1. Comparative example 2, however,exhibited the temperature-rising rate in the molding process was small,and the resin does not sufficiently flow, and a whitening phenomenon, amolding failure, of pre-preg sheet 101 was confirmed.

According to observation of peripheries of through-holes 3 of thecircuit boards of examples 1 to 3, no outflow of conductive paste 4 wasconfirmed visually.

In the boards of examples 1 to 3, the resistance of conductive paste 4filling through-holes 3 is stable even in pre-preg sheet 1 to becompressed at a small rate, hence providing a circuit board having ahigh quality. According to this embodiment, the board may be moldedafter the lamination and the compression are executed simultaneously,thus providing similar effects. Thus, the limitations to moldingfacilities having different capacity or function and meltingcharacteristics of the press intermediate material and the pre-pregsheet can be removed.

Therefore, a temperature profile corresponding to compression executedunder a non-continuous heating raising a temperature may be incorporatedinto a general pressing profile in the molding process, thus providingsimilar effects and increasing productivity.

Laminated body 81 is mounted in a heating device, such as a furnace, orin a heating/pressurizing device hot pressing, and is heated. Laminatedbody 81 is formed using a laminated device which is heated. At least oneof the heating/pressurizing device and the laminating device can be usedaccording to a capacity of a productive facility or a capacity ofproviding laminated body 81, thereby improving the productivity andstabilizing the quality.

According to this embodiment, pre-preg sheet 1 is made of a compositematerial including a non-woven fabric made of aromatic polyamide fiberand a thermosetting epoxy resin impregnated in the fabric. However,pre-preg sheet 1 at a B-stage may be manufactured by impregnating asubstrate made of woven fabric as a fiber sheet with a resin materialmainly containing thermosetting resin.

Pre-preg sheet 1 may be at a B-stage where a fiber sheet of woven fabricor non-woven fabric of glass fiber is impregnated with a resin materialmainly containing thermosetting resin. In particular, pre-preg sheet 1to be compressed at a small rate as shown in FIG. 1F has a largereffect. When pre-preg sheet 1 at the B-stage including fabric mainlymade of glass fiber and thermosetting resin impregnated in the fabric iscompressed at a small rate smaller than, e.g., 10%, the resistance ofconductive paste 4 in through-holes 3 is about 30% smaller than that ofthe comparative examples. The four-layer circuit board is explained asthe multi-layer circuit board according to the embodiment. However, amulti-layer circuit board having more than four layers provides similareffects. Thus, the present invention is not limited to the material andcondition of each examples. The laminating process, compressing process,and molding process according to the embodiment provides similareffects.

The limitations to the molding facility and the melting characteristicsof the intermediate material for pressing and the pre-preg sheet can beremoved. The present embodiment is effective especially for pre-pregsheet 1 having through-holes 3 filled with conductive paste 4, and theviscosity of the resin for securing conduction may not necessarily beadjusted precisely.

The laminating process and compressing process may be simultaneouslyexecuted and the molding process may then be executed, thus providingsimilar effects. The temperature profile corresponding to thecompressing process may be independently incorporated into a generalpress profile in the molding process, which provides similar effects.

According to the embodiment, the compressing process of heating andpressurizing the pre-preg sheet for a predetermined time at apredetermined pressure at a temperature close to the softeningtemperature of the impregnated resin is executed before the moldingprocess. Thus, adhesiveness and flatness of different kinds ofmaterials, such as the pre-preg sheet and the conductive circuit patternplaced on the metal foils or the circuit board, can be improved. Sincethe outflow of the conductive paste during the melting of the resin ofthe pre-preg sheet is reduced by lowering the temperature to not higherthan the softening temperature of the resin, the connection resistanceis stable.

Since pre-preg sheet 1 is heated, pressurized, and compressed for thepredetermined time at the predetermined pressure at the temperatureclose to the softening temperature of the impregnated resin before themolding, the adhesive force of different kinds of materials, such as thepre-preg sheet and the conductive circuit pattern placed on the metalfoils or the circuit board can be improved, and the board can beflattened.

Upon pre-preg sheet 1 being compressed before the molding, onlyconductive paste 4 is first pressurized selectively and contacts metalfoils 5 a and 5 b securely, and the resin of conductive paste 4 diffuseson surfaces of metal foils 5 a and 5 b to increase the contacting forcebetween conductive particles of conductive paste 4. This prevents theoutflow of conductive paste 4 due to the melting of the resin ofpre-preg sheet 1, and stabilizes the resistance of conductive paste 4.

The softening temperature of the thermosetting resin in conductive paste4 may be lower than that of the resin in pre-preg sheet 1. A rangeallowing the viscosity of the melting resin in pre-preg sheet 1 to belarge, namely, a range in which the resin is soft but hardly flows isprovided. Pre-preg sheet 1 can be easily compressed, and the contactingforce between the conductive particles of conductive paste 4 can beincreased. Further, deformation of pre-preg sheet 1 due to the meltingof the resin in pre-preg sheet 1 is reduced, thereby reducing resin flowand preventing the outflow of conductive paste 4.

The softening of the resin in conductive paste 4 is facilitated, and theviscosity of conductive paste 4 is close to the smallest, the resin inconductive paste 4 easily diffuses on the surface of metal foils 5 a and5 b, and the contacting force between the conductive particles ofconductive paste 4 can be increased.

Pre-preg sheet 1 is in the B-stage, thereby increasing the adhesiveforce against metal foils 5 a and 5 b.

The substrate of pre-preg sheet 1 may employ a non-woven fabric made ofaromatic polyamide fiber, thereby increasing a mechanical strength ofthe circuit board and reducing the weight of the circuit board. Thisallows through-hole 3 to have a small diameter, and stabilizes theresistance of conductive paste 4 filling through-hole 3, thus providinga the high-quality circuit board.

The substrate of pre-preg sheet 1 may employ woven fabric or non-wovenfabric of glass fiber. This material increases a mechanical andphysicochemical strength of the circuit board, and stabilizes theresistance of the conductive paste filling through-hole 3 in pre-pregsheet 1 especially when the sheet is compressed at a small rate in itsthickness direction, providing a high-quality circuit board.

Pre-preg sheet 1 is heated and pressurized for a predetermined time at apredetermined pressure at a heating temperature close to the softeningtemperature of resin 1 b impregnated into pre-preg sheet 1, preventingthe resin of pre-preg sheet 1 from flowing. This effect is remarkableespecially for pre-preg sheet 1 including substrate 1 a of woven fabricor non-woven fabric of glass fiber to be compressed at a relativelysmall compression rate not higher than 10%.

Pre-preg sheet 1 is compressed at a temperature close to the softeningtemperature of resin 1 b impregnated into pre-preg sheet 1, and than,laminated body 81 is molded at a temperature higher than thistemperature. This operation increases an adhesive force between thelayers and stabilizes the resistance of the conductive paste, thusproviding a high-quality circuit board.

When laminated body 81 is heated at a temperature close to the softeningtemperature of resin 1 b impregnated into pre-preg sheet 1 and thenheated at a temperature higher than this temperature, namely atemperature in the resin flowing/hardening range, the adhesive forcebetween the layers can be improved.

Laminated body 81 is heated at a temperature close to the softeningtemperature of resin 1 b impregnated into pre-preg sheet 1, is thenheated to a temperature higher than this temperature, namely atemperature in the resin flowing/hardening range, and is further heatedto a hardening temperature of resin 1 b higher than the temperature inthe resin flowing/hardening range, thereby increasing the adhesive forcebetween the layers, stabilizing the resistance of the conductive paste,and providing a high-quality circuit board.

Resin 1 b impregnated into pre-preg sheet 1 having a softening range inthe temperature range from 50° C. to 130° C. is soft but hardly flowswithin a temperature range from 65° C. to 85° C., and flows and startsbeing hardened within a temperature range from 85° C. to 140° C. Thisresin enables pre-preg sheet 1 to be easily compressed and molded andprovides sheet 1 including the resin flowing easily. This resinstabilizes the resistance of conductive paste 4 filling through-hole 3in pre-preg sheet 1 compressed at a small compression rate, providing ahigh-quality circuit board.

When laminated body 81 is temporarily taken out of the heating device orthe heating/pressurizing device, laminated body 81 may be cooled to havea temperature not higher than the softening temperature of resin 1 b.Then, the adhesive force between different kinds of materials, such aspre-preg sheet 1 and circuit patterns 6 a and 6 b on copper foils 5 aand 5 b or the circuit board can be increased, and the circuit board canbe flattened. Laminated body 81 is cooled to a temperature not higherthan the softening temperature of resin 1 b, thereby preventing theoutflow of conductive paste 4 during the melting of the resin ofpre-preg sheet 1 and stabilizing the resistance of the conductive paste.

Laminated body 81 is heated by the heating device, such as a furnace,and then, taken out to have its temperature decreased, thus improving aquality and productivity.

INDUSTRIAL APPLICABILITY

In a circuit board manufactured by a method according to the presentinvention, the resistance of a conductive paste filling a through-holeis stable even in a pre-preg sheet compressed at a small compressionrate.

1. A method of manufacturing a circuit board, comprising: preparing apre-preg sheet including a substrate and a resin impregnated in thesubstrate, the pre-preg sheet having a first surface and a secondsurface opposite to the first surface; placing a first metal foil on thefirst surface of the pre-preg sheet to provide a laminated body;compressing the laminated body at a temperature close to a softeningtemperature of the resin at a predetermined pressure; cooling thelaminated body to have a temperature not higher than the softeningtemperature of the resin after said compressing the laminated body atthe predetermined pressure; and bonding the first metal foil to thepre-preg sheet of the laminated body and hardening the resin.
 2. Themethod according to claim 1, wherein said compressing the laminated bodyat the predetermined pressure comprises compressing the pre-preg sheetat a compression rate smaller than 10%.
 3. The method according to claim1, wherein said placing the first metal foil on the first surface of thepre-preg sheet to provide the laminated body further comprised placing asecond metal foil on the second surface of the pre-preg sheet to providethe laminated body.
 4. The method according to claim 1, furthercomprising: forming a through-hole which penetrates the pre-preg sheetfrom the first surface to the second surface of the pre-preg sheet; andfilling the through-hole with a conductive paste.
 5. The methodaccording to claim 1, wherein the pre-preg sheet is in a B-stage and iscompressive.
 6. The method according to claim 1, wherein the substrateof the pre-preg sheet comprises a non-woven fabric of aromatic polyamidefiber.
 7. The method according to claim 1, wherein the substrate of thepre-preg sheet comprises a glass fiber.
 8. The method according to claim1, wherein said compressing of the laminated body at a temperature closeto a softening temperature of the resin comprises compressing thelaminated body at a first temperature, the first temperature being closeto the softening temperature of the resin, and wherein said bonding thefirst metal foil to the pre-preg sheet of the laminated body andhardening the resin further comprising: heating the laminated body at asecond temperature, the second temperature being higher than the firsttemperature; and heating the laminated body at a third temperature aftersaid heating the laminated body at the second temperature, the thirdtemperature being higher than the second temperature.
 9. The methodaccording to claim 1, wherein the resin of the pre-preg sheet has asoftening range from 50° C. to 130° C.
 10. The method according to claim4, wherein the conductive paste includes a conductive filler and athermosetting resin, and wherein a softening temperature of thethermosetting resin is lower than the softening temperature of the resinof the pre-preg sheet.
 11. The method according to claim 8, wherein thesecond temperature is within a flowing/hardening range of the resin. 12.The method according to claim 8, wherein the third temperature is ahardening temperature of the resin.
 13. The method according to claim 4,wherein the resin comprises a thermosetting resin.
 14. The methodaccording to claim 1, wherein said bonding the first metal foil to thepre-preg sheet of the laminated body and hardening the resin comprises:bonding the first metal foil to the pre-preg sheet; and hardening theresin after said cooling of the laminated body.
 15. The method accordingto claim 14, wherein said bonding of the first metal foil to thepre-preg sheet comprises bonding the first metal foil to the laminatedbody of the pre-preg sheet between said compressing of the laminatedbody and said cooling of the laminated body.